This same rule extends to multiple record definitions. In COBOL, multiple record definitions are redefinitions of the entire record area. This leads to the same complication that is encountered with REDEFINES: multiple definitions for the same data. So the compiler needs to select one definition to use.
Because the multiple record types can be different sizes, the compiler must use the largest one, so that it can cover all of the fields adequately. Thus, the compiler's rule is to use the fields in the largest record defined for the file. If more than one record is of the largest size, the compiler uses the first one.